I could not upgrade my work Confluence 7.19.X anymore, because the MySQL 8.x underneath was UTF-8 only.
So I found https://dba.stackexchange.com/questions/8239/how-to-easily-convert-utf8-tables-to-utf8mb4-in-mysql-5-5 - to whom the credit should go.
First - after stopping Confluence, snapshotting, backup etc:
I ran:
mysql -u root -p -s < gen_queries.sql > queries.sql
where the gen_queiries.sql is:
USE information_schema;
SELECT CONCAT("ALTER DATABASE `",table_schema,"` CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin;") AS _sql
FROM `TABLES` WHERE table_schema LIKE "confluence" AND TABLE_TYPE='BASE TABLE' GROUP BY table_schema UNION
SELECT CONCAT("ALTER TABLE `",table_schema,"`.`",table_name,"` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_bin;") AS _sql
FROM `TABLES` WHERE table_schema LIKE "confluence" AND TABLE_TYPE='BASE TABLE' GROUP BY table_schema, table_name UNION
SELECT CONCAT("ALTER TABLE `",`COLUMNS`.table_schema,"`.`",`COLUMNS`.table_name, "` CHANGE `",column_name,"` `",column_name,"` ",data_type,"(",character_maximum_length,") CHARACTER SET utf8mb4 COLLATE utf8mb4_bin",IF(is_nullable="YES"," NULL"," NOT NULL"),";") AS _sql
FROM `COLUMNS` INNER JOIN `TABLES` ON `TABLES`.table_name = `COLUMNS`.table_name WHERE `COLUMNS`.table_schema like "confluence" and data_type in ('varchar','char') AND TABLE_TYPE='BASE TABLE' UNION
SELECT CONCAT("ALTER TABLE `",`COLUMNS`.table_schema,"`.`",`COLUMNS`.table_name, "` CHANGE `",column_name,"` `",column_name,"` ",data_type," CHARACTER SET utf8mb4 COLLATE utf8mb4_bin",IF(is_nullable="YES"," NULL"," NOT NULL"),";") AS _sql
FROM `COLUMNS` INNER JOIN `TABLES` ON `TABLES`.table_name = `COLUMNS`.table_name WHERE `COLUMNS`.table_schema like "confluence" and data_type in ('text','tinytext','mediumtext','longtext') AND TABLE_TYPE='BASE TABLE';
After that, I opened the queries.sql file, and inserted at the top:
SET foreign_key_checks = 0;
and at the bottom:
SET foreign_key_checks = 1;
And then ran:
mysql -u root -p < queries.sql
Took 4-5 hours to complete, and should end without errors....
Thats it - start Confluence and all is green and it can be upgraded